Any loss of PheH activity can result in the accumulation of Phe and its neurotoxic metabolites, as well as in the depletion of tyrosine and methionine, the latter being needed for the formation of catecholamine neurotransmitters such as dopamine, as well as for protein synthesis. The outcome of such metabolic changes can often be neurological disease.
Several hundred different mutations in the PheH gene have been characterised over the entire length of the protein. Many of these have been recorded and annotated in the PheH (or PAH) database, http://www.pahdb.mcgill.ca/, which provides expression analysis data. These mutations result in a wide spectrum of phenotypes that range in severity from mild hyperphenylalaninaemia (HPA) (serum Phe levels of 400-600 mmol/L) with a low risk of impaired cognitive development, to phenylketonuria (PKU) (MIM:261600) characterised by high serum Phe levels (>1200 mmol/L), which if left untreated causes severe progressive mental retardation, epilepsy and microcephaly; these symptoms can largely be prevented by a Phe-restricted diet. In addition to mutations in PheH gene, about 2% of cases arise from mutations in the enzymes that synthesise the BH4 cofactor. HPA and PKU are autosomal recessive disorders, with an average incidence of 1/10,000 in Caucasian populations.
Most of the mutations in PheH are point (missense) mutations that most frequently result in protein misfolding. Misfolded monomers tend to associate through exposed hydrophobic surfaces to form aberrant oligomers that can irreversibly form large protein aggregates. Cells have a quality control system to remove mutant polypeptides so as to prevent the potentially damaging effects of protein aggregates, usually through ubiquitinylation. All forms of the mutant enzyme, including the misfolded polypeptide, aberrant oligomers and large protein aggregates, are subjected to an increased rate of proteolytic degradation, which reduces the intracellular levels of mutant PheH and consequently decreases Phe catabolism leading to HPA or PKU diseases.
A small number of missense mutations result in decreased enzyme affinity for either substrate or BH4 cofactor binding, or affect allosteric regulatory properties of the enzyme. For example, some PheH deficiencies are responsive to the oral administration of BH4 cofactor. BH4‑responsive PheH deficiencies all show residual catalytic activity, but have a reduced affinity for BH4 cofactor binding resulting from mutations in the catalytic domain. The administration of BH4 is thought to increase the binding of the cofactor, as well as to protect the mutant enzyme from misfolding and degradation by increasing protein stability, thereby acting as a chemical chaperone.
Other genes may be involved in the severity of the PKU phenotype. For instance, the enzyme monoamine oxidase type B (MOAB) can degrade phenylethylamine, a toxic Phe metabolite that can cause brain injury. It is thought that MOAB might be a modifier gene in PKU, because different levels of phenylethylamine have been found in PKU patients with similar levels of Phe.